1. Field of the Invention
The present invention relates to radars. More specifically it relates to a radar's sensitivity timing control circuit.
2. Description of the Related Art
A radar system operates by transmitting a pulse of radio frequency (RF) energy through the atmosphere to a target. The RF energy is reflected by the target and is received by the radar antenna. As the radar signal propagates through the atmosphere, it is attenuated. As a result, signals received from targets that are relatively close to the radar will be attenuated less than signals that are received from targets relatively far from the radar. To compensate for this attenuation, a sensitivity timing control (STC) circuit is used. The STC circuit feeds the received signal to amplifiers that have a gain which is varied over time. The circuit compensates for the atmospheric attenuation by providing less gain or amplification to signals received from targets that are close to the radar, and by providing more gain to signals received from targets that are relatively far from the radar.
The typical STC circuit includes amplifiers with variable gain. The amplifier which is used most widely in aviation electronics is the MC 1350 manufactured by Motorola. The 1350 performs well, but its parameters vary from part-to-part, from lot-to-lot and over temperature. In many radar systems, the gain introduced by the STC circuit is not controlled using a feedback control loop. This leaves the STC gain dependent on the variations in the amplifiers' parameters.
Radar systems with open loop STC circuits suffer from several problems. In the manufacture of the STC circuit, each circuit must be adjusted to compensate for the variations in the amplifiers that occur from device-to-device. In addition, once the unit is fielded, the gain of the STC circuit varies over temperature because there is no feedback control loop to compensation for the variations introduced by the amplifiers.